Wind turbines consist of a fixed tower which raises an electric generator connected to a rotor by mechanical means above the ground. The rotor is formed by a hub or base joining one or several blades which are responsible for transforming the kinetic energy of the wind into rotational movement of the rotor. The turbine is generally placed on a frame, which is able to move with respect to the tower supporting it such that it allows the rotor to be oriented in the direction of the wind. One manner of orientation can be letting the frame (forming part of what is usually called the nacelle of the wind turbine) freely rotate (leeward orientation), which frame would act like a wind vane. However, to obtain better functioning, a mechanical system is normally used to move the frame with the rotor to a desired orientation (windward orientation).
This rotation of the frame on a horizontal plane is called “yawing”.
The mechanical systems for orienting a wind turbine with the wind generally consist of a gear ring, normally fixed to the tower, and a motor normally fixed to the frame with the generator and which makes the frame rotate by means of a gear wheel meshing with the gear ring. The problem posed by this process is that once the frame is located in one orientation by means of the motor, it does not remain immobile, but rather gusts of wind can exert a momentum which moves the frame. Therefore the motor must be constantly running, and the gear teeth of the ring and of the wheel undergo loads in both directions, causing allowances and the deterioration of the gear teeth, which requires costly repairs.
To solve this problem, several nacelle braking systems are placed. One example can be, seen in Japanese patent application JP-A-08 082 277, disclosing a hydraulic brake joined to the frame which acts on a disc joined to the tower.
Another example can be seen in U.S. Pat. No. 4,966,525 and U.S. Pat. No. 5,035,575, using the yawing system by means of a gear ring fixed to the tower and motors fixed to the frame, meshing with the ring. To block the movement of the frame, two motors are installed on the frame which, when it is desired for the latter to remain still, act with an opposite rotational direction, and which act in the same direction in order to make the frame rotate. Electromagnetically released security brakes applied on the shafts of the motors are also included. The swivel joint between the frame and the tower is by means of, a ball bearing.
Another type of yawing system is disclosed in U.S. Pat. No. 5,178,518. In this case, the frame rotates freely, it is not mechanically operated, and the brake systems this time are for preventing excessive yawing speeds in the frame (for both the yawing and pitching), and are controlled by the centrifugal force.
European patent EP-B1-0 945 613 discloses a yawing system where the frame rotates on a ball bearing. It includes a continuous passive braking system preventing the motor from being forced to operate due to small gusts of wind. To this end, it comprises friction plates on the frame which are pushed against the support ring in the tower by means of springs, on which ring the frame rotates. These plates can be placed in the upper track or in the lower track of the ring, or in a radial direction on a cylindrical surface joined to the tower. The braking force is passive, i.e. it is adjusted during the assembly of the nacelle, and this braking is always operating either against small gusts of wind or against the desired movement of orientation of the rotor. This patent also claims an embodiment including electrically driven disc braking devices (active braking device).
A yawing system by means of a gear ring in the tower and several motors in the frame, with a braking system by means of active and passive brakes, is also known. However, these brakes act on a surface which is also the frame support surface. This forces seeking a balance on that surface between the need for a good lubrication to facilitate the yawing movement of the frame and the need for a surface with a high coefficient of friction to favor the operation of the brakes.
The known systems have several drawbacks. One of them is that the braking systems do not have enough retention capacity against strong gusts of winds. Increasing the passive braking systems would require also increasing the power necessary to orient the rotor. Increasing the known active braking systems requires a larger friction surface or a greater force for acting on the elements joined to the tower (friction disc or the like), but the weight and space in the upper part of the tower forces making much sturdier towers, increasing the cost of the wind turbine. Braking by means of drive motors of an acceptable size in the towers themselves is neither enough to prevent movements due to strong gusts of wind.
Another problem is that the elements of the yawing system of the wind turbine, especially the gear ring, the nacelle support ball bearings and the braking systems undergo significant loads, which leads to the frequent need for repairs or replacements. A large braking system would require the disassembly of the nacelle in order to repair it.
On the other hand, the gear teeth of the gear ring may undergo wear and tear and/or breakage, and in the known systems, a breakdown in the bearings supporting the nacelle allowing the rotation on the tower, or in the ring where the motor meshes, requires the disassembly of the nacelle, taking it to ground level so as to carry out the repair.
To solve these problems, the yawing system for wind turbines of the invention has been developed.